Unexpected irregular reflection signal from a high-finesse cavity

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teen_physicist
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I am observing an irregular, aperiodic noise pattern in the reflection signal of a high-finesse optical cavity (finesse ≈ 20,000). The cavity is normally operated using a standard Pound–Drever–Hall (PDH) locking configuration, where an EOM provides phase modulation.

The signals shown in the attached figures were recorded with the modulation turned off. Under these conditions, when scanning the laser frequency across a cavity resonance, I expected to observe a simple reflection dip. Instead, the reflected signal shows a noisy, irregular structure that does not resemble a clean resonance feature.

I have briefly surveyed the literature and found discussions of possible contributors such as:
  • coupling to higher-order transverse modes,
  • free-running frequency jitter of the laser,
  • coherence or interference effects involving multiple cavity modes.
However, the example signals presented in those works look qualitatively different from what I observe here, so it is unclear whether the same mechanisms are responsible.


Has anyone encountered similar behavior in a high-finesse cavity when the PDH modulation is disabled?
What physical mechanisms could lead to such an irregular reflection signal, and what experimental steps would you recommend to diagnose or mitigate this noise?
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The "like" wasn't really what I intended. "Interesting" is what I meant. Hope someone can shed some light on your problem.
 
sbrothy said:
The "like" wasn't really what I intended. "Interesting" is what I meant. Hope someone can shed some light on your problem.
Is the spectrum analyser set to a very narrow bandwidth so that the problem is exaggerated? You seem to obtain a bandwidth of just a few Hertz. Could the cavity be subject to acoustic or mechanical vibration?
 
sbrothy said:
The "like" wasn't really what I intended. "Interesting" is what I meant. Hope someone can shed some light on your problem.
Thanks!!!
 
tech99 said:
Is the spectrum analyser set to a very narrow bandwidth so that the problem is exaggerated? You seem to obtain a bandwidth of just a few Hertz. Could the cavity be subject to acoustic or mechanical vibration?
Ideally, I don’t think this behavior should appear, even for a very narrow bandwidth oscilliscope. The issue is also present in the PDH error signal itself, where a similar irregular structure is visible and clearly degrades the locking performance.

The cavity is mounted on an air-floated optical table, so at least it seems unlikely that the dominant contribution is table-borne vibration.
 
The reason I found your problem interesting is that I, myself, might need to use an oscilloscope in order to profile a laser for my Raman spectrometer project. I haven't got much to contribute technically but I'm interested in any serious answers, so I sort of bumped the thread (which I know is frowned upon by the mods).

I may be asking a dumb question but what are you trying to do?
 
sbrothy said:
The reason I found your problem interesting is that I, myself, might need to use an oscilloscope in order to profile a laser for my Raman spectrometer project. I haven't got much to contribute technically but I'm interested in any serious answers, so I sort of bumped the thread (which I know is frowned upon by the mods).

I may be asking a dumb question but what are you trying to do?
No worries. Any support is welcomed!! In short term, I want to the PDH locking and get my laser linewidth narrow down to a couple kHz. In long term, I am planning to couple serval lasers into this cavity to do the Rydberg excitation, which requires narrow linewidth for higher order energy levels.